[Concurrency] Properly set task create flags in TaskGroup.startTaskSynchronously

rdar://147907609

When starting tasks synchronously on task groups, the task create flags for isSynchronousStart and isDiscardingTask were always set to `false`. With this change the former will always be `true` and the latter conditionally `true` for discarding task groups.

Author: drexin

stdlib/public/Concurrency/Task+startSynchronously.swift.gyb

@@ -230,8 +230,8 @@ extension Task where Failure == ${FAILURE_TYPE} {
       inheritContext: true,
       enqueueJob: false,
       addPendingGroupTaskUnconditionally: false,
-      isDiscardingTask: false,
-      isSynchronousStart: false
+      isDiscardingTask: ${'true' if 'Discarding' in GROUP_TYPE else 'false'},
+      isSynchronousStart: true
     )
 
     let (task, _) = Builtin.createAsyncTask(flags, operation)
@@ -249,4 +249,4 @@ extension Task where Failure == ${FAILURE_TYPE} {
 internal func _startTaskOnMainActor(_ task: Builtin.NativeObject)
 
 @_silgen_name("swift_task_startSynchronously")
-internal func _startTaskSynchronously(_ task: Builtin.NativeObject)
+internal func _startTaskSynchronously(_ task: Builtin.NativeObject)